There are two types of ISUP services: basic and supplementary. Basic service provides the support for establishing connections for circuits within the network. These circuits can be audio circuits for voice transmission or data circuits for any digital information, voice, or data. Supplementary services are all other circuit-related services, which typically encompass message transport after a call path is established.

In addition to the two types of services, ISUP uses two methods for end-to-end signaling. End-to-end signaling is the process of sending circuit-related information from one exchange to a distant exchange. These two exchanges may be adjacent to each other or across Local Access Transport Areas (LATAs).

The method currently used for passing signaling information to the distant exchange is called the pass-along method. With the pass-along method, the signaling information moves from one exchange to the next. All subsequent information related to the same circuit is then passed using the same path that was used to send the initial call-setup information. This, of course, means that information must follow the same ISUP hops as the setup messages, which is not the most efficient method of routing.

The alternative method is called the SCCP method and uses the services of the SCCP protocol to route the message through the network. When using the SCCP protocol, the information does not have to follow the same path as the call-setup information. In fact, it can follow any path, provided the final destination is the same.

The...